Intro

Sometimes even high-end 8½-digit DMM is not a best tool, since these meters still have limited input resistance and leakage current to earth. Sometimes it’s a deal breaker, and small nV-meter with battery power only would be a better use in differential bridge measurements. This is particularly useful for high-voltage high-resistance bridges and such.

For this and few other reasons, we got Fluke Model 845AB null-meter for $250 USD. Let’s see what makes it work.

Electrical Specifications for Model 845

Description

Manufacturer specification

Input Voltage Range

1 microvolt to 1000 volt DC end scale in 19 ranges, using X1 and X3 progression

0-1 volt, one side at chassis ground; to 0.5% of end scale. Output can be operated as a DC amplifier with a <=gain of 120 db

Stability of Zero

Better than 0.15 microvolt/hour, Better than 0.3 microvolt/day

Temperature Coefficient of Zero

Less than 0.1 microvolt/degree C from 15 °C to 35 °C

Zero Control Range

±5 microvolt minimum

Overload Protection

Up to 1100 volts dc may be applied on any range. Typical recovery time is 4s

Input Power

Rechargeable battery or 115/230 VAC +/- 10%, 50 to 440 Hz

Battery life

40 hours operation on full charge, trickle charge on line power

Weight

10 pounds

Size

8 inches high by 8 inches wide by 9 inches deep

Fluke 845AB were sold new for $2500 USD during their lifetime and were considered a Primary Standards Laboratory-grade electronic instruments. Fluke 845AR was also used by industry as part of other instrumentation, such as resistance bridge measurement system and DC voltage standards.

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Tear-down and construction

It’s rather simple concept, with just few parts in each block, with easy to understand operation, unlike most of modern test instrumentation gear filled with lots of digital logic, microcontrollers, firmware and so on.

Metal panels are easy to remove, allowing easy access to inner circuitry. Only board accessible easily is the mains power/re-charge circuit with 84Hz multivibrator and transformers.

If one to trust label on battery pack, it’s rather fresh and unit was serviced before.

Chopper amplifier

I was really expected to see infamous neon tubes, light guides and photo-resistor chopper amplifier, but was greeted instead with alien-looking green PCB with micropower regulator Linear LT1120CN8 and few HP 2731 optocouplers(?) !

LT1120 dated with 1993 week 52. Is it that optically-isolated Bilateral Analog chopper, which mentioned in Fluke 845AB manual’s note in 7/1993 print?

Indeed it is. I’ll be reverse engineering this circuit later on so we can know exactly what we dealing with here, and evaluate possibility of future improvements.

Here are some additional photos of chopper module board:

Transistors used are general purpose NPN’s 2N5089 in TO92 package with hFE about 400 and 30V maximum voltage.
Optocouplers are Fairchild H11F1, which are good FET switches with decent linearity and low offsets.

And connection of three wires from high impedance nodes on base board:

Repair

After initial inspection and testing next issues were spotted:

Meter does not read anything in BATOPR mode. It’s just dead on zero, like no power is supplied, even though BATCHK shows good battery

In LINEOPR mode meter is unable to zero on ranges below 30µV

Rest of ranges seems to be working as expected. Input resistance measured by 3458A is 100 MΩ on high voltage ranges, 10 MΩ on mV ranges and ~250K&Omega on ranges below 1 mV.

Main supply voltage rails +15 and -15 were measured at 17.65 and -17.8 VDC respectively.

Meter cannot zero on lowest ranges

Green PCB with chopper assembly has blue multi-turn trimmer pot, accessible from the side. This trimmer adjustment allowed shifting zero, and now meter can be zeroed on lowest 1 µV range.

This was an easy repair.

Replacement of all electrolytic caps

As usual with equipment with age over 10, replacement of all electrolytic capacitors should be a must-have maintenance procedure. This saves trouble of cleaning electrolyte leaks and further destruction. If you think that’s rare malfunction, be sure to check our Keithley Model 2001 experience with no less than 7 meters affected by damage from old capacitors.

There is plenty space inside, so usual good quality Chemi-Con radial capacitors were used instead of original axial type. I used 470µF 63V instead of original 470µF 25V and 6.3V caps.

Everything inside is very clean, not a single mark of dust, as there is no vent holes or fan in this device.

Some decent resistors are installed directly on range switch, including large high voltage 90 MΩ monster with golden leads. Most critical wiring protected by Teflon insulation and soldered to Teflon PCB standoffs.

Calibration procedure

Manual section 4-20 have simple calibration procedure. Model 845 should be calibrated once every year. It is recommended that if component replacement is performed, the Model 845 should be re-calibrated. Test and alignment points are illustrated in instruction manual.